refactor: split core/ui/apps targets and add unit tests

Move all business logic into liblivecaption_core (no Qt dependency) and
the widgets into liblivecaption_ui, leaving two executables: the caption
window and the CLI, which no longer links Qt Widgets. Abstract the ASR
backend behind IAsr so the streaming policy and the live pipeline can be
tested with a scripted fake backend instead of a real model.

把全部业务逻辑下沉到不依赖 Qt 的 liblivecaption_core，界面独立成
liblivecaption_ui，拆出悬浮窗与命令行两个可执行程序（CLI 不再链接 Qt
Widgets）；并用 IAsr 抽象推理后端，使流式策略与实时管线可脱离真模型
用假实现做单元测试。测试还抓到一个真 bug：段恰好结束在 chunk 边界时
只提交了稳定部分，最后几个词被丢掉，已修。

tests/fakeasr.h

@@ -0,0 +1,75 @@
+#pragma once
+
+#include <cctype>
+#include <cstddef>
+#include <string>
+#include <vector>
+
+#include "iasr.h"
+
+// 可脚本化的假推理后端：让流式策略与实时管线可以脱离真模型测试。
+//
+// dropLastTokens 用"空格分词"模拟词表：丢掉最后 N 个词，并保留原有格式
+// （这点很重要 —— 续写开头那个空格是有意义的）。
+class FakeAsr : public lc::IAsr {
+public:
+    // 每次 streamAppend 依次返回这里的文本；用完后返回空串
+    std::vector<std::string> scripted;
+
+    // 记录（供断言用）
+    std::vector<std::string> prefixesSeen;
+    std::vector<size_t>      samplesSeen;
+    int                      resets = 0;
+
+    int sampleRate() const override { return 16000; }
+
+    lc::AsrResult transcribeWithPrefix(const float *, size_t,
+                                       const std::string &) override
+    {
+        lc::AsrResult r;
+        r.ok = true;
+        if (call_ < scripted.size()) {
+            r.raw = scripted[call_];
+            r.transcript = lc::parseAsrOutput(r.raw);
+        }
+        ++call_;
+        return r;
+    }
+
+    lc::AsrStreamStep streamAppend(const float *, size_t nSamples,
+                                   const std::string & prefix, int) override
+    {
+        prefixesSeen.push_back(prefix);
+        samplesSeen.push_back(nSamples);
+
+        lc::AsrStreamStep s;
+        s.ok = true;
+        if (call_ < scripted.size()) s.newText = scripted[call_];
+        ++call_;
+        s.newTokens      = 1;
+        s.encodeSeconds  = 0.001;
+        s.inferSeconds   = 0.001;
+        return s;
+    }
+
+    void streamReset() override { ++resets; }
+
+    std::string dropLastTokens(const std::string & text, int dropLast) const override
+    {
+        if (dropLast <= 0 || text.empty()) return text;
+
+        const auto isSpace = [](char c) { return std::isspace((unsigned char) c) != 0; };
+
+        size_t pos = text.size();
+        for (int i = 0; i < dropLast; ++i) {
+            while (pos > 0 && isSpace(text[pos - 1])) --pos;      // 跳过尾部空白
+            while (pos > 0 && !isSpace(text[pos - 1])) --pos;     // 跳过一个词
+            if (pos == 0) return {};
+        }
+        while (pos > 0 && isSpace(text[pos - 1])) --pos;
+        return text.substr(0, pos);
+    }
+
+private:
+    size_t call_ = 0;
+};